Custom Windows - What is common practice when specifying?

Created by Vicki Marshall, Modified on Fri, 3 Mar, 2023 at 2:41 PM by Vicki Marshall

Region: All States

Assessor Question:

I'd like some information on what is common practice when specifying custom windows? If the plans don’t specify a supplier, can I simply put custom windows from a variety of suppliers that have the most appropriate U and SHGC?

For example, the lowest U value default window for Timber double glazed has quite a low SHGC compared to the custom windows available so can I choose custom versions to substitute. And if some of those suppliers do not have each of the various opening types that I require, can I specify the windows from a variety of suppliers just based on the U and SHGC.

TechLink Response:

Customs windows cannot be used when a window manufacture is not stated on the drawings.
Default windows must be used. If the architect / builder / developer has chosen a window manufacture, have the drawing set amended to state the window manufacture.

The NatHERS Tech note states;

If a custom window is specified on the design documentation that is not available in the NatHERS custom window library, assessors must model:

default window with the specified opening type;
or an available custom window that meets all of the following parameters:
identical opening type (e.g. fixed, awning, casement, sliding) to the window specified on the documentation;
and a total window system U-value equal to, or greater than, the window specified on the documentation (e.g. if the U-value of the specified custom window is 3.0, the modelled window selected could be 3.5);
and a total window system Solar Heat Gain Coefficient (SHGC) +/-5% of the window specified on the documentation.